Option clicking on iPhoto's Rotate button will cause the image to rotate clockwise (normal clicking results in an counter-clockwise rotation). Quicker than using the menu bar or shortcuts. This may be documented but there again I never read the documentation.

[Editor's note: The iPhoto help is actually very robust. If you select Help, then click on "More..." in the News section, you'll get a list of keyboard shortcuts, including this option-click on Rotate trick. As another example, did you know that option double-clicking a photo will open it in a separate, resizable window? There's more, too, so give the Help pages a quick read-through!]